Coated proppants are often used in hydraulic well fracturing to increase production rate of the well. The commercial “standard” coatings are typically a form of phenolic thermoset coating. Partially cured phenolic proppants are typically used in low temperature wells (i.e., those having bottom hole temperature of less than about 150° F. (66° C.)) which typically exhibit low crack closure stresses (e.g., 2000-6000 psi). The theory behind their use is that the residual reactivity of the partially cured phenolic coating in conjunction with a surfactant activator (which acts like a plasticizer) and the existence of water found in most wells will permit the coating to soften and flow, thereby allowing the proppants to consolidate and form interparticle bonds during the “shut-in” period. The formation temperature of the downhole conditions is supposed to complete the curing reactions in situ in the propped formation. An external activator fluid is used to soften the outer surface of these partially cured coated proppants in an effort to encourage consolidation and interparticle bonding. The activator itself raises, however, additional issues of compatibility with the fracturing and breaker fluids as well as the possibility of adverse effects on the continued conductivity of the proppant packed fractured strata.
For high temperature wells, such as those with a bottom hole temperature above about 200° F. (93° C.), both partially cured phenolic coated proppants (with an activator) or precured phenolic coatings are often used. In the case of the precured coated proppant, the fracture/crack closure stresses are often above 6,000 psi are used as the main mechanism for holding proppant within the cracked strata.
In practice, however, a variety of factors can adversely affect the performance and usefulness of the partially cured, phenolic coatings. The most important of these is premature curing of the partially cured phenolic resin in the coating due to exposure to high temperatures before introduction into the fractured strata. Even the elevated, above-ground, temperatures found on loading docks and in shipping containers can be enough to effect curing of the coating long before it is desirable. This is particularly an issue that comes into play when using partially-cured phenolic coatings in deep high temperature wells. In low temperature applications, the partially cured phenolic coatings simply take too long to cure to create bond strength. Bond strength can be developed in a reasonable time frame if the activator is used in conjunction with the partially-cured, coated proppant. However, using the activator requires the addition to be metered into the fracturing fluid in a controlled manner at the right time. This increases the complexity of the fracturing treatment. Even if the activator is added at the proper concentration and time, there still remain the issues with fracturing fluid compatibility and the aforementioned reduced fracture conductivity.
Two published patent applications discuss the use of isocyanates for proppant coatings. Tanguay et al. 2011/0297383 presents examples of high temperature proppant coatings made of a polycarbodiimide coating on sand. The coating is said to be made from the reaction of a monomeric isocyanate and a polymeric isocyanate. The catalyst is a phosphorous-based catalyst exemplified in example 1 by 3-methyl-1-phenyl-2-phospholene oxide.
Tanguay et al. 2012/0018162 relates to a polyamide imide proppant coating for high temperature applications. The examples have a description of the use of polymeric diphenylmethane diisocyanate, trimellitic anhydride, one of three different types of amines, triethylamine as a catalyst, an adhesion promoter and a wetting agent. The coating/reaction process described lasts about 10 minutes followed by a post-cure heating of 1-3 hours.
Recently, it has been discovered that cured, commercially acceptable, coatings can be applied to proppants using the polyurethane or polyurea reaction products of polyols and isocyanates. The details of these processes are disclosed in co-pending U.S. patent application Ser. No. 13/099,893 (entitled “Coated and Cured Proppants”); Ser. No. 13/188,530 (entitled “Coated and Cured Proppants”); Ser. No. 13/626,055 (entitled “Coated and Cured Proppants”); Ser. No. 13/224,726 (entitled “Dual Function Proppants”); Ser. No. 13/355,969 (entitled “Manufacture of Polymer Coated Proppants”); and Ser. No. 13/837,396 (entitled “Proppant With Polyurea-Type Coating”), the disclosures of which are herein incorporated by reference. Commercially available proppants that use such coatings are available under the designations PEARL and GARNET from Preferred Sands, Inc. Such polyurethane and polyurea-based proppant coatings are economically and environmentally desirable for a number of reasons. Importantly, each acts like a fully cured coating for purposes of handling, shipping and introduction into a fractured field yet exhibit the inherent ability to form interparticle bonds under downhole temperatures and pressures for enhanced conductivity and proppant flowback control.
Despite the benefits found by the interparticle bonding seen in recent proppant coatings, there exists a continuing need in the industry for a proppant coating that will controllably form interparticle bond strength at a wide variety of the expected downhole temperature and pressure conditions yet will not be compromised in forming such interparticle bond strength by premature exposure to elevated or high temperatures.